The cellular fatty acid compositions of 368 strains of Campylobacter species or Campylobacter-like organisms were determined by gas-liquid chromatography. Most of the strains (339) were placed in one of three groups based on differences in fatty acid profiles. Group A contained Campylobacter jejuni (97%) and most C. coli (83%) strains and was characterized by the presence of a 19-carbon cyclopropane fatty acid (19:0 cyc) and 3-hydroxytetradecanoic acid (3-OH-14:0). Group B included all C. laridis and some C. coli (17%) strains; its profile was similar to that of group A, except that 19:0 cyc was absent. Group C contained C. fetus subsp. fetus and C. fetus subsp. veneralis and was characterized by the presence of 3-OH-14:0 and 3-hydroxyhexadecanoic acid (3-OH-16:0) and the absence of 19:0 cyc. Twenty-nine isolates were placed in four additional groups. Group D included the type strain of "C. cinaedi" and 14 other isolates, which were differentiated by the presence of dodecanoic acid (12:0), 3-hydroxydodecanoic acid (3-OH-12:0), and 3-OH-16:0 and the absence of hexadecenoic acid (16:1) and 3-OH-14:0. Group E contained the type strain of "C. fennelliae" and two additional isolates, which were differentiated by the presence of a 16-carbon aldehyde and a 16-carbon dimethylacetyl and the absence of 16:1. Group F included the type strain and one reference strain of C. cryaerophila and six human isolates whose phenotypic characteristics were similar to those of this species; this group was distinguished by the presence of two isomers of 16:1, tetradecenoic acid (14:1), and 3-OH-14:0. Group G included three stains of C. pyloridis and was characterized by the presence of 19:0 cyc, 3-OH-16:0, and 3-hydroxyoctadecanoic acid (3-OH-18:0) and by the absence of 16:1 and 3-OH-14:0.